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THE CIVIL WORKS FOR THE BARRIERS OF THE VENICE FLOOD PREVENTION SYSTEM
FOUR MOBILE BARRIERS WILL BE ABLE TO SEPARATE THE LAGOON FROM THE ADRIATIC SEA DURING TIDE EVENTS – THE FLAP GATES ARE ANCHORED TO SEVERAL CONCRETE CAISSONS THAT SUPPLY POWER AND COMPRESSED AIR VIA HINGES – GATE CAISSONS ARE LARGE PRECAST R.C. STRUCTURES FEATURING HIGH ACCURACY AND DURABILITY
Location: Venice, Italy
Client: Venezia Nuova Consortium, concessionary of the Ministry for Infrastructure and Transport - Venice Water Board
Cost of total Works: 905,000,000 €
Services: Detailed design of barrier caissons at the four lagoon inlets
The protection of Venice from flooding tides is achieved by isolating the lagoon from the sea through four mobile barriers crossing the inlets of Lido, Malamocco and Chioggia for a total extension of about 1600 m. An artificial island separates into two barriers the Lido closure.
Any barrier consists of several steel flap gates (variable in number between 18 and 21) supported on 6-7 r.c. foundation caissons. The outer dimensions of the housing structures depend largely on the size of the gates, operational spaces and water depth of the channels. The largest caissons are 60 m long (when supporting 3 gates), 11.5 m high and almost 50 m wide, for a total weight of about 20,000 t.
The closing structures of the barrier function as connection between the intermediate immersed section (360-420 m long and everywhere below the seabed) and the banks; they consist of two caissons emerging from the sea water level, each composed of a high precast cellular structure (up to 27.5 m) that will be finished on site, after completion of sinking and ballasting operations, with the construction of three additional floors.
The caissons feature several ballast compartments to receive water and heavy ballast.
Effects on hydrodynamics at the lagoon inlets, along with environmental and navigation consequences, were reduced by adopting a construction method based on the prefabrication of the barrier elements (caissons, gates and hinge-connector units) in dedicated areas, away from the bed of the channels: temporary dry docks built in the future refuge harbors for Treporti and Chioggia and a precast area on a reclaimed land for Malamocco and S. Nicolò. The caissons built on the embankment have the peculiarity to be moved over rails by means of a trolley system, from the precast area to the syncrolift® platform, where each of them will be lowered and floated in water.
When the caissons of one barrier will be completely built and equipped, they will be floated and, by means of an installation barge, transported above the special trench prepared in the related inlet, sunk in their final position and permanently ballasted. Anchoring of the caissons inside the trench is ensured by grouting the gap underneath the caisson base and backfilling the remaining space between the sides of the caissons and the sheet piles. This potential hydraulic path must be well sealed because, when the barriers will be closed, up to 2 m of hydrostatic head could be generated between the sea and lagoon sides of the gates.
More than a hundred immersed tunnels have been built worldwide to provide road and rail connections, but Venice caissons, while being constructed and assembled using the same technique, are unique in their features and functionality.
The gate caisson is a cellular structure characterized by 4-5 horizontal slabs and a grid of vertical partitions spaced about 5 m in both directions (average thickness of about 40 cm). Size and complexity of the structure requires several casting phases (from 15 to 23) with both horizontal and vertical construction joints always protected by adequate waterproofing systems. Concrete cast for each batch varies between 100 and 450 m3, except for the bottom slab, where about 1000 m3 have to be cast in a single step.
Many a steel equipment has to be fixed to the strongest r.c. slabs and walls of the caisson, often with an accuracy that is not usual for concrete structure, enabling:
- Temporary transformation of the caisson into a vessel, closing temporarily the tunnels with steel bulkheads, providing small towers on the top of the caisson for inspection and access along with strong points and bollards for tow and handling maneuvers of the floating caisson;
- Construction of a closed unit (barrier), across the shoulders, using the technique of the immersed tunnels: by leveling the sunk caisson with high accuracy with huge steel pins protruding the bottom slab of the caisson, pulling each caisson against the one previously placed by mating provisions, closing the Gina seal system installed at the end of each caisson on precise steel frames;
- Connecting each gate to the caisson by two hinge-connector units. The socket part of this connection (female element) is a steel box grouted on a thick r.c. slab and anchored by prestressing 10 large diameter bars.
The concrete used to build the overall precast barrier caissons was about 220,000 m3 for an average rate of steel bars of about 300 kg per cubic meter of reinforced concrete. 15-20% of this reinforcement is AISI 316L stainless steel, used where both water and air may penetrate in the concrete cover.
The barrier is designed for a service life of 100 years and this requirement implied careful considerations for: choice of materials, corrosion protection systems, assessment of the behavior under cyclic loading (fatigue), development of cracking phenomena and settlement evolution for the sequence of caissons (joint tightness).
To improve the strength of the concrete against chloride entry, the opening of the concrete microstructure has been controlled, addressing a mix design able to minimize:
- the permeability or diffusivity of concrete microstructure
- the heat development at early stages
- the proneness to cracking
Based on the concrete data achieved through laboratory tests, a FEM analysis of the caissons have been performed with:
- thermo-mechanical 3D models using TNO DIANA software;
- non-linear analysis considering constitutive law of concrete, creep and shrinkage;
- calculation of evolution of hydration and temperature during hardening of concrete;
- calculation of internal stress field, taking into account the evolution of possible cracking during the hardening process.
A number of successful trial castings (mock-up) representing a part of the typical cellular structure has been executed in the yard areas.
The barrier system is highly innovative and has therefore requested numerous in-depth studies, through testing models (mathematical and physical) and prototypes. An important part of these investigations have concerned hydrodynamic effects generated by waves and tidal currents on the barrier components, both during construction and in operation.
Technical Drawings & Renderings
Venice lagoon and the three inlets temporary closed by the mobile barriers
Gate caissons in construction (Malamocco)
Lido Treporti dry dock
Malamocco precast area on embankment
Gate caissons completed (during the flooding operation of the dock)
Installation of Treporti gate caisson - 1
Installation of Treporti gate caisson 2
Close-up of the caisson head (steel bulkhead and Gina joint)